The present invention has immediate application to the aquaculture industry, and more particularly to an apparatus and method for mechanically separating solid waste particles from a liquid stream and automatically ejecting them from the liquid environment. This invention has immediate application to commercial aquaculture and aquatic research. It may find application in other fields, including but not limited to general wastewater treatment and landscaping industries.
AQUACULTURE is the science, art, and business of cultivating marine or freshwater aquatic organisms, and is one of the fastest growing sectors of United States agriculture. Most of this growth is in the intensive fish culture sector, where wastewater is renewed by solid waste separation and other treatments before being recycled back to the culture system. These intensive recycle systems are conservative in terms of water use and land area requirements, and are often housed for further environmental control. Such intensification has allowed for aquaculture expansion into geographical areas having low water yields and marginally adequate climate and source-water temperature.
Intensive aquaculture wastewater contains high concentrations of particles in the form of fecal material and uneaten food and dissolved nitrogenous waste in the form of ammonia, nitrite, and nitrate. The concentration of these pollutants must be substantially reduced prior to water re-use or discharge into the environment. The specific gravity of fish fecal material ranges from 1.05 to 1.20, and wasted feed is considerably denser. Since particle densities exceed that of water, they settle from the water column under low velocity conditions.
Intensive recycle aquaculture systems typically employ multiple cylindrical tanks with diameters ranging from 5 to 50 feet, and depths from 2 to 5 feet. The major advantage of cylindrical tanks as applied in fish culture is that they are virtually self-cleaning. Water flowing into the fish tanks is directed tangentially to the tank wall at the tank outer radius. With proper water inflow structure and flow velocity, solid waste particles settle by centrifugal and gravitational forces to the bottom-center of the tank. The movement of fish within the tanks enhances the migration of settled particles toward the tank center. The congregated particles are continuously drawn through a bottom-center drain to separation systems prior to recycle or discharge to the environment.
Centrifugal-gravitational solid waste removal studies on fish culture tanks have shown that tanks having diameter-to-depth ratios greater than 10:1 do not self-clean well. Since tank depths greater than five feet are not commonly used for aquaculture, tank diameters rarely exceed 50 feet, and are typically 30 feet or less. The use of many smaller tanks offers advantages over the use of a few larger tanks. If one small tank system fails due to equipment failure or disease outbreak, the percent of the loss is less than in the case of a large tank failure. The assumption here is that each fish tank has its own dedicated solid waste separation circuit, hydraulically separate from other tanks.
A recent advance in fish tank technology is the “dual flow,” or “double drain,” system. Dual flow fish tanks have two water treatment circuits, one for solid waste separation and one for dissolved nitrogenous waste conversion. Nitrogenous waste conversion reactors are often referred to in the aquaculture industry as nitrification reactors.
Dual flow fish tank technology has drawn attention to the need for compact solid waste settling separators. A dual flow system is comprised of a low-flow recycle circuit (around 20% of the total system water volume per hour) through the bottom-center drain for solid waste removal and a high flow recycle circuit (around 80% of the total system water volume per hour) through a tank sidewall port for dissolved nitrogenous waste processing in a nitrification bioreactor. Nitrogenous waste production dictates the flow rate required through the bioreactor. The main revelation of the dual flow concept is that by un-coupling the solid waste circuit from the bioreactor circuit, the solid waste separator can be physically downsized, and can operate at a very low water flow rate. This indicates the usefulness of gravitational separators for solid waste removal that are capable of handling high particulate concentrations at low flow rates.
In high-density aquaculture systems that treat water for system reuse, solid wastes accumulating in gravitational separators or filters will cause water quality deterioration if the accumulated waste is not removed frequently. Over time, captured or settled particles can re-enter the flow stream, resulting in an accumulation of suspended particles and dissolved organics in the culture system. This occurs by several mechanisms, including shear forces in pressurized screen or media filters and by bacterial action. Toxic gases including carbon dioxide and hydrogen sulfide are produced in sludge-laden filters and gravitational settlers. Buoyancy created by these gases can cause re-suspension of settled particles. Stagnant zones of sludge can harbor fish parasites and pathogens. For these reasons, it is widely recognized that solid waste should be removed from solid waste accumulators as frequently and as thoroughly as possible. In the case of gravitational separators, cleaning and sludge removal is accomplished by backwashing, draining, and/or manual washing of internal vessel surfaces.
THE PRIOR ART includes swirl separators, hydro-cyclones, deceleration basins, and off-line settling cones. Cylindrical settling separators separate waste particles from flow streams by the same means as cylindrical fish tanks, namely, centrifugal-gravitational forces. In aquaculture applications, these units typically remove particles eighty microns in diameter and larger, representing 80% of the total particulate loading. This is satisfactory for some fish species, and for most species raised in systems using high water dilution rates. Prior art settling separators are non-pressurized, meaning that the water surface in the separator is at atmospheric pressure. Vessel geometry imperfections result in a certain degree of distortion of the centrifugal flow pattern. The distortions can manifest as errant water currents that can entrain lower-density particles and prevent them from settling. Some other solid waste accumulators typically used for aquaculture recycle systems, including up-flow bead filters (pressurized) and micro-screen filters (non-pressurized), can remove particles of 30 micron diameter and larger, but are relatively expensive to buy, install, and/or operate. Foam fractionators (flotation devices) are often used in conjunction with particle settling devices to remove finer particles (100 micron diameter and less), dissolved organics, and surfactants from aquaculture systems. Use of effective settling separators with foam fractionators successfully removes all particle sizes from aquaculture systems.
Centrifugal-gravitational particle settler hydraulic loading rates range from three to 5 gallons per minute per square foot of horizontal separator area. For a given particle size and density, removal efficiency is a function of separator vessel geometry, resulting flow dynamics, and hydraulic loading rates. Cylindrical centrifugal separators are more effective than longitudinal-flow basins, which have hydraulic loading rates of about 1 gallon per minute per square foot. This means that they take up four times the floor space as centrifugal-gravitational settlers.
In intensive recycle systems, fish tank water is typically recycled at a rate of one tank volume per hour. For a thirty-foot fish tank of 4-foot depth, with 20% of the recycle flow passing through the solid waste separator, the flow through the solids separator would be about 50 gallons per minute (gpm). For a centrifugal-gravitational settling separator operating at a hydraulic loading rate of 5 gpm per square foot of horizontal separator area, the separator would be only four feet in diameter and use 12.6 square feet of floor space. A comparable rectangular settling basin would have a footprint of 50 square feet.
Manual cleaning of sludge accumulation in settling basins and cones involves opening drain valves and wiping cone sidewalls. Automation of large sludge drain valves is expensive, and forceful flushing can only occur if there is adequate head pressure. This can be a disadvantage on flat sites.
The prior art review reveals the need for a low cost, high efficiency particle separator that offers the simplicity of a centrifugal-gravitational settling cone and an automated system for frequent ejection accumulated sludge and for cleaning internal separator surfaces. Such a device will lend itself well to multi-tank systems, each tank having its own separator, but with the full system operated by a single centralized compressed air system with automated sludge ejection control. This application offers savings on equipment, maintenance, labor, and electrical costs.